1. Field of the Invention
This invention relates to diversity transmission systems, and more particularly, to a diversity system for transmitting digital data over short distances between a mobile transmitter and a fixed receiver.
2. Description of the Prior Art
When a signal is transmitted from a mobile transmitter at one point to a receiver at another point in a complex environment where propagation is primarily by re-radiation from a multitude of scattering points, several rays of the transmitted signal will experience different diverse paths before arriving at a receiver antenna.
When the signal received is composed of two rays transversing different path lengths such that one path is odd multiples of one-half wave length longer than the other, a dip in the received signal will occur. When the two paths differ in length by multiples of one wave length, an increase in the signal received will occur. As the distance between the two points of communication is increased, the spatial separation of the dips or nulls approach one wavelength. Thus, such path diversity gives rise to a succession of peaks and nulls in the signal detected at the receiver as the distance between the transmitter and receiver changes, resulting in fading in the received signal, distortion of the received intelligence, or a combination of both depending upon the differences in the path lengths traversed by the transmitted signal and the nature of the intelligence.
Diversity transmission systems have been employed to circumvent problems in communication between two remote points caused by signals from the transmitting location travelling over paths of different lengths recombining at the receiving location. The most common forms of diversity reception are space and frequency.
One example of a system utilizing a form of frequency diversity is disclosed in the U.S. Pat. No. 3,361,970 to H. Magnuski. The system, which is intended to provide a communication link between computing machines and apparatus situated in different cities, employs two separate transmitting-receiving channels separated by a large frequency to avoid fading and coincidence problems. In a disclosed embodiment, carrier signals at different frequencies are modulated by signals from a single modulating supply and transmitted to a receiving location. The transmitted signals are detected by a pair of receivers, each tuned to a different frequency, and a signal selector at the output of the receivers selects the strongest signal received.
A system employing space diversity is disclosed in the U.S. Pat. No. 3,348,150 to Atal et al. In this system, a single RF carrier frequency is used, and voice signals are separated into several redundant channels and used to modulate the carrier frequency. The resultant signals are supplied to separate widely spaced antennas for transmission to a receiver. The receiver demodulates the signals and produces a combination of the filtered speech signals.
While various frequency and space diversity systems have been proposed in the prior art, each system is usually tailored to a specific application, and generally some compromise must be made to obtain satisfactory operation for such application. Thus, a diversity system designed for one application may not be at all suitable for another application. Considering, for example, a remote meter reading system, in such system, coded interrogate signals are transmitted from an interrogate source, which may be fixed or mobile, to transponders associated with utility meters. Each transponder responds to a selected interrogate signal to transmit to the interrogate source a reply signal indicating a reading of the associated meter. When a mobile interrogate source is employed, fading may occur in the signal received by the transponder as the separation between the interrogate source and the transponder changes. This is caused by the multiplicity of propagation paths taken by the signal as a result of the electrically obscured environment of the transponder location.
Although a frequency diversity system of the type shown by Magnuski, for example, would avoid the effects of fading due to multipath propagation, the need to provide two receivers and a complex combining network at each meter installation would make such system uneconomical for remote meter reading applications where the large number of receivers required dictate a need for a simple low cost receiver.
The system shown by Atal et al is practical for the transmission of voice signals where only a small portion of the spectrum is needed for intelligibility. However, such system would not be suitable for the transmission of digital data over short distances as in a remote meter reading system.
Therefore, it would be desirable to have a diversity transmission system including a simple low cost receiver which enables reliable transmission of digital data between a mobile transmitting source and the receiver, as in a remote meter reading system.